1. Field of the Invention
This invention relates to a method of obtaining a hologram using a photoresist used for an in-line type computer hologram interferometer (hereinafter simply referred to an interferometer), and to a hologram exposure apparatus.
2. Description of the Prior Art
An interferometer is quite often used for measuring optical surfaces of non-spherical shapes and for measuring known shapes maintaining high precision. In particular, an interferometer of the in-line type is used for measuring the optical surfaces of an axis-symmetrical lens and the shape of the metal mold surfaces for press-molding the lenses. In the interferometer of the in-line type, a reference light and a light from an object to be measured are coaxially arranged so that a pattern of the hologram describes concentric circles. When the object light has spherical waves, it is allowed to measure deep shapes compared to the pattern pitch of the hologram and the optical system becomes almost common to the reference light and the object light, making it possible to suppress error caused by the optical system. The hologram used for the in-line interferometer has heretofore been obtained by a method according to which a concentric pattern of the hologram is found by a computer and is drawn using an X-Y plotter followed by taking a picture thereof, or by a method according to which a photoresist is exposed using an electron beam drawing apparatus instead of using the X-Y plotter or taking a picture of it.
According to the method which uses the X-Y plotter and takes a picture, however, the concentric pattern of the hologram tends to develop distortion, coma and spherical aberration due to such factors as precision of the X-Y plotter, precision for setting at the time of taking a picture, illumination and exposure conditions, and processing conditions for developing. Moreover, the processing becomes complex. According to the method of using an electron beam drawing apparatus, on the other hand, the drawing precision is considerably high for a narrow range of drawing. When the drawing range is expanded, however, leakage of magnetic flux increases from the electromagnetic lens and an extended exposure time is required. Therefore, the drawing position drifts causing the drawing precision to decrease. If it is attempted to eliminate this defect, the apparatus becomes bulky and expensive. Generally, the electron beam drawing apparatus is expensive by its own nature and has a drawing range of a square having a side of about 5 mm, and is not suited for obtaining a hologram.